1. Field of the Invention
The present invention relates to frequency converters and radio communications systems employing the frequency converter and, more particularly, to a frequency converter and a radio communication, in which a received radio frequency signal is converted into an intermediate frequency signal and an intermediate frequency signal to be transmitted is converted into a radio frequency signal.
2. Description of the Related Art
There is a growing demand for a high-speed network as data communications rapidly gain widespread use. High-speed communications provided by wired networks are still very expensive for individual users, and local radio communications networks providing a low-cost service are now actively studied and developed. Such a local radio network uses radio frequency bands, such as quasi millimeter waves (20 GHz-30 GHz) or millimeter waves (30 GHz-300 GHz), capable giving a high antenna gain with a small antenna. In the local radio network, a hub station, for example, installed in a telephone exchange station, provides high-speed two-way data communications service or local TV phone service to a plurality of (user) subscriber stations within a predetermined coverage area.
When radio communications are performed using radio frequency signals in the quasi millimeter band or the millimeter band, an intermediate frequency signal of several tens to several hundreds of megahertz, rather than radio frequency signals, is subjected to a receiving process including an isolation decoding step, a transmitting process including coding and synthesis steps, and an amplification of signals. The cost of a circuit arrangement required for the receiving and transmitting processes and the signal amplification is thus reduced. FIG. 16 shows a frequency converter, which converts a radio frequency signal into an intermediate frequency signal or an intermediate frequency signal into a radio frequency signal.
The frequency converter is installed in a subscriber station, for example, and receives radio waves, transmitted from a hub station, through its receiving antenna 1001. Out of the radio waves received by the receiving antenna 1001, radio frequency RF(RX) signals of interest for reception in a plurality of frequency channels in a range of 22.6 GHz-23.0 GHz are extracted by a bandpass filter 1002.
The radio frequency RF(RX) signal extracted through the bandpass filter 1002 is amplified to an appropriate level by a low noise amplifier 1003, and is then mixed with a TX/RX local oscillation frequency signal LO1, for example, a 21 GHz signal, by a mixer 1004.
The local oscillation frequency signal LO1 is generated by a phase-locked oscillator 1100.
The phase-locked oscillator 1100 includes a phase-locked loop including a counter circuit 1102, a frequency comparator 1103, a loop filter 1105, and a voltage-controlled oscillator 1106, and frequency multipliers 1107 and 1109.
In the phase-locked oscillator 1100, a signal output by the voltage-controlled oscillator 1106 is frequency-divided, for example, by 175, by the counter circuit 1102. The frequency comparator 1103 compares a signal output by the counter circuit 1102 to a reference signal, for example, a 10 MHz reference signal supplied by a reference oscillator 1204 employing a highly stable crystal oscillator. A voltage, corresponding to the difference between the two signals, is then amplified by the loop filter 1105 in appropriate frequency characteristics. The voltage output from the loop filter 1105 is fed back to a control input of the voltage-controlled oscillator 1106.
A 1.75 GHz signal output by the voltage-controlled oscillator 1106 is frequency-multiplied by four times by the frequency multiplier 1107, becoming a 7.0 GHz signal. The remaining signals contained in the output of the frequency multiplier 1107 are filtered out by a bandpass filter 1108.
The signal output by the bandpass filter 1108 is further frequency-multiplied by three times by the frequency multiplier 1109, becoming a 21.0 GHz signal. The remaining signals contained in the output of the frequency multiplier 1109 are filtered out by a bandpass filter 1110.
In this way, the phase-locked oscillator 1100 results in the signal having the local-oscillation frequency LO1 (21 GHz) at the same frequency accuracy level as that provided by the highly stable reference oscillator 1204.
The local-oscillation frequency signal LO1 is output by the bandpass filter 1110, and is amplified by an amplifier 1112, and is then received by the RX mixer 1004.
An output of the mixer 1004 contains the frequency components of the sum of, and the difference between, the radio frequency RF(RX) signal and the local-oscillation frequency LO1 signal. The difference between the two signals, i.e., a signal in an intermediate frequency band IF1(RX) of 1.6 GHz-2.0 GHz, is extracted by the bandpass filter 1005, is amplified by an amplifier 1006, and is fed to an RX mixer 1007.
The RX mixer 1007 mixes the signal in the intermediate frequency band IF1(RX) and a local-oscillation frequency LO2 signal, for example, a 1.1 GHz signal supplied by a phase-locked oscillator 1200.
The local-oscillation frequency LO2 signal is generated by the phase-locked oscillator 1200.
The phase-locked oscillator 1200 includes a phase-locked loop including a counter circuit 1202, a frequency comparator 1203, a loop filter 1205, and a voltage-controlled oscillator 1206, and the high-accuracy reference oscillator 1204 employing a crystal oscillator.
In the phase-locked oscillator 1200, a signal output by the voltage-controlled oscillator 1206 is frequency divided, for example, by 110, by the counter circuit 1102. The frequency comparator 1203 compares a signal output by the counter circuit 1202 to a reference signal, for example, a 10 MHz reference signal supplied by the reference oscillator 1204. A voltage, corresponding to the difference between the two signals, is then amplified by the loop filter 1205 in appropriate frequency characteristics. The voltage output from the loop filter 1205 is fed back to a control input of the voltage-controlled oscillator 1206.
In this way, the phase-locked oscillator 1200 results in the signal having the local-oscillation frequency LO2 (1100 MHz) at the same frequency accuracy level as that provided by the highly stable reference oscillator 1204.
The output of the RX mixer 1007 contains frequency components of the sum of, and the difference between, the signal in the intermediate frequency band IF1(RX) and the local-oscillation frequency LO2 signal. The difference between the two signals, i.e., a signal in an intermediate frequency band IF2(RX) of 500 MHz-900 MHz, is extracted through the bandpass filter 1008.
The signal in the intermediate frequency band IF2(RX), picked up by the bandpass filter 1008, is amplified by an amplifier 1009, is fed to a diplexer 1010, and is then fed to a demodulator (not shown) via an IF cable.
The signal in the radio frequency band RF(RX) thus received is converted into a signal in an appropriate intermediate frequency band IF2(RX).
The signal in the intermediate frequency band IF2(TX), for example, 10 MHz-60 MHz, supplied by a modulator (not shown), is received from the diplexer 1010 via the IF cable, is amplified by an amplifier 1012, and fed to a TX mixer 1013. The RX intermediate frequency IF2 (RX) and the TX intermediate frequency IF2(TX) are assigned in separate frequency ranges.
The TX mixer 1013 mixes the signal in the intermediate frequency band IF2(TX) with the signal having the local-oscillation frequency LO2 output by the phase-locked oscillator 1200.
The output of the TX mixer 1013 contains the signals of the sum of, and the difference between, the signal in the intermediate frequency band IF2(TX) and the local-oscillation frequency LO2 signal. The signal of the sum of the two signals, i.e., a signal in an intermediate frequency band IF1(TX) of 1.11 GHz-1.16 GHz, is extracted by a bandpass filter 1014.
The signal in the intermediate frequency band IF1(TX), extracted by the bandpass filter 1014, is amplified by an amplifier 1015, and is then fed to a TX mixer 1016.
The TX mixer 1016 mixes the signal in the intermediate frequency band IF1 (TX) with the signal in the local-oscillation frequency LO1 output by the phase-locked oscillator 1100.
The output of the TX mixer 1016 includes the signal components of the sum of, and the difference between, the signal in the intermediate frequency band IF1(TX) and the signal having the local-oscillation frequency LO1. The signal of the sum of the two signals, i.e., a signal in a radio frequency band RF(TX) of 22.11 GHz-22.16 GHz, is extracted by a bandpass filter 1017.
The signal in the radio frequency band RF(TX), extracted by the bandpass filter 1017, is amplified to an appropriate level, and is fed to a transmitting antenna 1019. The corresponding radio wave is then transmitted through the transmitting antenna 1019 to the hub station.
In this way, a signal in the appropriate intermediate frequency band IF2(TX) is frequency converted into a signal in the radio frequency band RF(RX) and is then transmitted.
The conventional frequency converter employs the phase-locked oscillators 1100 and 1200 to respectively generate a plurality of signals in the local-oscillation frequencies LO1 and LO2, shared by the transmitter part and the receiver part.
The phase-locked oscillators having a crystal oscillator working in the quasi millimeter band or the millimeter band are generally costly. They are complex and bulky, requiring a substantial maintenance cost, and consume much power, requiring a substantial operating cost. As a result, if the frequency converter is built in a subscriber station in a local radio network (a radio communications system), the cost of each individual subscriber""s subscriber station increases much.
When the local-oscillation frequency is generated using the phase-locked oscillator, the frequency accuracy inevitably degrades in proportion to the ratio N of the frequency of the output signal to the frequency of the reference signal in the crystal oscillator, and a phase noise inevitably increases in proportion to the square of N. As the local-oscillation frequency becomes higher, the attained accuracy level of the output frequency and the level of the phase noise are limited more.
For example, when the frequency accuracy of the 10 MHz reference oscillator is +/xe2x88x9210 Hz (1E-6) and the phase noise is xe2x88x92120 dBc/Hz at an offset of 1 kHz, N=21 GHz/10 MHz=2100 at a frequency of 21 GHz. The attained frequency accuracy level is +/xe2x88x9221 kHz, and the phase noise level is xe2x88x9254 dBc/Hz at an offset of 1 kHz because 20 log 2100=66 dB.
If the local-oscillation frequency becomes higher, a frequency multiplier maybe required. The frequency multiplier can work as a source of unwanted radiated signals, degrading spurious characteristics of the frequency converter.
These degraded characteristics lead to a drop in the utilization of frequencies when communications are performed using frequency division multiplexing.
Accordingly, it is an object of the present invention to provide a frequency converter which results in an improved accuracy level of frequency and implements a compact and low-cost design by employing a phase-locked oscillator for a relatively low local-oscillation frequency only. It is also an object of the present invention to provide a radio communications system which results in an improved utilization of frequencies and lightens the cost imposed on a subscriber by incorporating the frequency converter.
To achieve the above object, a frequency converter of the present invention, in one aspect, converts a signal in a first frequency band to a signal in a second frequency band by successively mixing the signal in the first frequency band with a plurality of local-oscillation signals having different frequencies. The frequency converter includes a phase-locked loop, wherein the phase-locked loop generates a local-oscillation signal having a low frequency, of the plurality of local-oscillation signals, based on an intermediate frequency beacon signal, which is generated by mixing a predetermined radio frequency beacon signal with the local-oscillation signal.
The phase-locked loop generates the local-oscillation signal having the low frequency, of the plurality of local-oscillation signals, based on the intermediate frequency beacon signal, which is generated by mixing the predetermined radio frequency beacon signal with the local-oscillation frequency signal. Even if a frequency offset and a phase noise take place in a local-oscillation signal having a high frequency, of the plurality of local-oscillation signals, the frequency offset and the phase noise are compensated for or canceled out when the signal in the first frequency band is mixed with the local-oscillation signal having the low frequency. Specifically, even if the phase-locked loop is used in the frequency converter to generate the low local-oscillation frequency signal only, the frequency offset and the phase noise taking place in the remaining high frequency local-oscillation signals are compensated for or canceled out. A high frequency accuracy thus results. This arrangement reduces the number of bulky, costly and power-consuming phase-locked oscillators, typically used in the quasi millimeter band or the millimeter band. A simplified, compact frequency converter is thus provided, reducing both installation and operating costs. The overall frequency accuracy of the frequency converter, employing the phase-locked loop, is improved. Furthermore, since a frequency multiplier is dispensed with, the spurious characteristics of the frequency converter are improved.
In a preferred embodiment of the present invention, the phase-locked loop may generate the local-oscillation signal having the low frequency, of the plurality of local-oscillation signals so that a difference between the frequency of the intermediate frequency beacon signal and a predetermined reference frequency becomes zero.
In a preferred embodiment of the present invention, the phase-locked loop may generate the local-oscillation signal having the low frequency, of the plurality of local-oscillation signals so that the intermediate frequency beacon signal is synchronized with a signal having a predetermined frequency.
The phase-locked loop generates the local-oscillation signal having the low frequency, of the plurality of local-oscillation signals so that the intermediate frequency beacon signal is synchronized with the predetermined frequency signal. Even when a frequency offset and a phase noise take place in a signal local-oscillation having a high frequency, of the plurality of local-oscillation signals, the frequency offset and the phase noise are compensated for or canceled out when the signal in the first frequency band is mixed with the local-oscillation signal having the low frequency, of the plurality of local-oscillation signals.
In a preferred embodiment of the present invention, the phase-locked loop may synchronize the local-oscillation signal having the low frequency, of the plurality of local-oscillation signals, with the intermediate frequency beacon signal.
The phase-locked loop generates the local-oscillation signal having the low frequency, of the plurality of local-oscillation signals, in synchronization with the intermediate frequency beacon signal. Even when a frequency offset and a phase noise take place in a local-oscillation signal having a high frequency, of the plurality of local-oscillation signals, the frequency offset and the phase noise are compensated for or canceled out when the signal in the first frequency band is mixed with the local-oscillation signal having the low frequency, of the plurality of local-oscillation signals.
In a preferred embodiment of the present invention, the signal in the first frequency band may be a radio frequency signal while the signal in the second frequency band may be an intermediate frequency signal.
The construction of the receiver part of the frequency converter for converting the radio frequency signal into the intermediate frequency signal is simplified and is made compact. The installation cost and operating cost of the converter are thus reduced.
In a preferred embodiment of the present invention, the signal in the first frequency band may be an intermediate frequency signal while the signal in the second frequency band may be a radio frequency signal.
The construction of the transmitter part of the frequency converter for converting the intermediate frequency signal into the radio frequency signal is simplified and is made compact. The installation cost and operating cost of the converter are thus reduced.
A radio communications system of the present invention, in another aspect, includes a hub station and at least one subscriber station radio linked to the hub station. The subscriber station communicates with the hub station by converting a signal in a first frequency band to a signal in a second frequency band by successively mixing the signal in the first frequency band with a plurality of subscriber station local-oscillation signals having different frequencies. The hub station transmits, to the subscriber station, a beacon signal having a radio frequency that is generated by mixing a signal having a hub station local-oscillation frequency with a beacon signal having a predetermined hub station intermediate frequency. The subscriber station includes a phase-locked loop, wherein the phase-locked loop generates a local-oscillation signal having a low frequency, of the plurality of subscriber station local-oscillation signals used in the subscriber station, based on a subscriber station intermediate frequency beacon signal, which is generated by mixing a predetermined radio frequency beacon signal, transmitted from the hub station, with the subscriber station local-oscillation signal.
The hub station transmits, to the subscriber station, the radio frequency beacon signal into which the beacon signal having the hub station intermediate frequency and the signal having the hub station local-oscillation frequency are mixed. On the other hand, the subscriber station communicates with the hub station by successively mixing the signal in the first frequency band with the plurality of subscriber station local-oscillation signals having the different frequencies. The phase-locked loop generates a local-oscillation signal having a low frequency, of the plurality of local-oscillation signals, based on the intermediate frequency beacon signal, which is generated by mixing the predetermined radio frequency beacon signal with the local-oscillation frequency signal. Therefore, even when a frequency offset and a phase noise take place in a local-oscillation signal having a high frequency, of a plurality of local-oscillation signals, the frequency offset and the phase noise are compensated for or canceled out when the signal in the first frequency band is mixed with the local-oscillation signal having the low frequency. Specifically, even if the phase-locked loop is used in the frequency converter to generate the low local-oscillation frequency signal only, the frequency offset and the phase noise taking place in the remaining high frequency local-oscillation signals are compensated for or canceled out. A high frequency accuracy thus results. This arrangement reduces the number of bulky, costly and power-consuming phase-locked oscillators, typically used in the quasi millimeter band or the millimeter band. A simplified, compact frequency converter is thus provided, reducing both installation and operating costs and thereby lightening the burden on the subscriber. The overall frequency accuracy of the frequency converter, employing the phase-locked loop, is improved. Furthermore, since a frequency multiplier is dispensed with, the spurious characteristics of the frequency converter are improved.
In a preferred embodiment of the present invention, the phase-locked loop may generate the subscriber station local-oscillation signal having the low frequency, of the plurality of subscriber station local-oscillation signals so that a difference between the frequency of the subscriber station intermediate frequency beacon signal and a predetermined reference frequency becomes zero.
In a preferred embodiment of the present invention, the phase-locked loop may generate the subscriber station local-oscillation signal having the low frequency, of the plurality of subscriber station local-oscillation signals so that the subscriber station intermediate frequency beacon signal is synchronized with a signal having a predetermined frequency.
In the subscriber station, the phase-locked loop generates the subscriber station local-oscillation signal having the low frequency, of the plurality of subscriber station local-oscillation signals so that the subscriber station intermediate frequency beacon signal is synchronized with the predetermined frequency signal. Even when a frequency offset and a phase noise take place in a subscriber station local-oscillation signal having a high frequency, of the plurality of subscriber station local-oscillation signals, and in the signal in the hub station local-oscillation frequency, the frequency offset and the phase noise are compensated for or canceled out when the signal in the first frequency band is mixed with the subscriber station local-oscillation signal having the low frequency, of the plurality of local-oscillation frequency signals.
In a preferred embodiment of the present invention, the phase-locked loop may synchronize the subscriber station local-oscillation signal having the low frequency, of the plurality of subscriber station local-oscillation signals, with the subscriber station intermediate frequency beacon signal.
The phase-locked loop generates the subscriber station local-oscillation signal having the low frequency, of the plurality of subscriber station local-oscillation signals, in synchronization with the subscriber station intermediate frequency beacon signal. Even when a frequency offset and a phase noise take place in a subscriber station local-oscillation signal having a high frequency, of the plurality of subscriber station local-oscillation signals, the frequency offset and the phase noise are compensated for or canceled out when the signal in the first frequency band is mixed with the subscriber station local-oscillation signal having the low frequency, of the plurality of subscriber station local-oscillation signals.
In a preferred embodiment of the radio communications system of the present invention, the signal in the first frequency band may be a radio frequency signal while the signal in the second frequency band may be an intermediate frequency signal.
The construction of the receiver part of the subscriber station of the radio communications system for converting the radio frequency signal into the intermediate frequency signal is simplified and is made compact. The installation cost and operating cost of the radio communications system are reduced.
In a preferred embodiment of the radio communications system of the present invention, the signal in the first frequency band may be an intermediate frequency signal while the signal in the second frequency band may be a radio frequency signal.
The construction of the transmitter part of the subscriber station of the radio communications system for converting the intermediate frequency signal into the radio frequency signal is simplified and is made compact. The installation cost and operating cost of the radio communications system are reduced.
In a preferred embodiment of the present invention, a predetermined frequency range used by the hub station and the subscriber station is divided into a plurality of frequency channels.
Since the radio communications system employs the phase-locked loop for the low frequency local-oscillation signal only, the overall frequency accuracy of the system is improved and the utilization of frequencies is improved.
A frequency converter of the present invention, another aspect, converts a signal in a first frequency band to a signal in a second frequency band by successively mixing the signal in the first frequency band with a plurality of local-oscillation signals having different frequencies, wherein an intermediate frequency signal, into which a spread spectrum reference signal having a predetermined frequency and a local-oscillation frequency signal are mixed, is despread to result in a reference signal, and based on the resulting reference signal, a local-oscillation signal having a low frequency, of the plurality of local-oscillation signals having the different frequencies is generated.
In a preferred embodiment of the present invention, the frequency of the local-oscillation signal having the low frequency, of the plurality of local-oscillation signals, may be determined based on the level of the reference signal that is obtained by despreading the intermediate frequency signal.
In the frequency converter, the frequency of the local-oscillation signal having the low frequency, of the plurality of local-oscillation signals, is set based on the level of the reference signal that is obtained by despreading the intermediate frequency signal. When the reference signal is correctly despread, the reference signal obtained through the despreading process exceeds a predetermined threshold level. The intermediate frequency then is regarded as being coincident with the transmission frequency when the reference signal is spread. Even when a frequency offset and a phase noise take place in a subscriber station local-oscillation signal having a high frequency, of a plurality of local-oscillation frequency signals, the frequency offset and the phase noise are compensated for or canceled out, and a high accuracy level of frequency is achieved. Since the reference signal is overlapped in the same frequency band as that of data signal, through the spread spectrum modulation, with almost no effect incurred on the data signal, no band outside the frequency band for the data signal is consumed in a useless fashion, and generally limited frequency bands are efficiently utilized. Since there is no need for crystal oscillators or the like, a simple and compact design may be promoted further, and the installation and operating costs of the converter are reduced.
In a preferred embodiment of the present invention, the signal in the first frequency band may be a radio frequency signal while the signal in the second frequency band may be an intermediate frequency signal.
The construction of the receiver part of the frequency converter for converting the radio frequency signal into the intermediate frequency signal is simplified and is made compact. The installation cost and operating cost of the converter are reduced.
In a preferred embodiment of the present invention, the signal in the first frequency band may be an intermediate frequency signal while the signal in the second frequency band may be a radio frequency signal.
The construction of the transmitter part of the frequency converter for converting the intermediate frequency signal into the radio frequency signal is simplified and is made compact. The installation cost and operating cost of the converter are reduced.
A radio communications system of the present invention, in yet another aspect, includes a hub station and at least one subscriber station radio linked to the hub station, wherein the subscriber station communicates with the hub station by converting a signal in a first frequency band to a signal in a second frequency band by successively mixing the signal in the first frequency band with a plurality of subscriber station local-oscillation signals having different frequencies. The hub station transmits, to the subscriber station, a spread spectrum signal that is generated by spreading a predetermined reference signal and then by mixing a hub station local-oscillation frequency signal with the spread spectrum reference signal. A subscriber station local-oscillation signal having a low frequency, of the plurality of subscriber station local-oscillation signals used in the subscriber station, is generated from a reference signal that is obtained by despreading a subscriber station intermediate frequency signal which results from mixing the spread spectrum signal transmitted from the hub station and the subscriber station local-oscillation frequency signal.
In a preferred embodiment of the present invention, the frequency of the subscriber station local-oscillation signal having the low frequency, of the plurality of subscriber station local-oscillation signals, may be determined based on the level of the reference signal that is obtained by despreading the intermediate frequency signal.
In the radio communications system, the hub station transmits, to the subscriber station, the spread spectrum signal that is obtained by subjecting the predetermined reference signal to the spread spectrum process, and by mixing the spread spectrum reference signal with the hub station local-oscillation frequency signal. To communicate with the hub station, the subscriber station converts the signal in the first frequency band to the signal in the second frequency band, by successively mixing the signal in the first frequency band with the plurality of subscriber station local-oscillation signals having different frequencies. In the frequency conversion process, the subscriber station local-oscillation signal having the low frequency, of the plurality of subscriber station local-oscillation signals, is set in frequency so that the level of the reference signal that is obtained by despreading the subscriber station intermediate frequency signal exceeds a predetermined threshold. When the reference signal is correctly despread, the reference signal obtained through the despreading process exceeds the predetermined threshold level. The subscriber station intermediate frequency then is regarded as being coincident with the transmission frequency when the reference signal is spread. Even when a frequency offset and a phase noise take place in a subscriber station. local-oscillation signal having a high frequency, of a plurality of local-oscillation signals, the frequency offset and the phase noise are compensated for or canceled out, and a high accuracy level of frequency is achieved. Since the reference signal is overlapped in the same frequency band as that of data signal, through the spread spectrum modulation, with almost no effect incurred on the data signal, no band outside the frequency band for the data signal is consumed in a useless fashion, and generally limited frequency bands are efficiently utilized. Since there is no need for crystal oscillators or the like, a simple and compact design may be promoted further, and the installation and operating costs of the converter are reduced.
In a preferred embodiment of the present invention, the signal in the first frequency band may be a radio frequency signal while the signal in the second frequency band may be an intermediate frequency signal.
The construction of the receiver part of the radio communications system for converting the radio frequency signal into the intermediate frequency signal is simplified and is made compact. The installation cost and operating cost of the radio communications system are reduced.
In a preferred embodiment of the present invention, the signal in the first frequency band may be an intermediate frequency signal while the signal in the second frequency band may be a radio frequency signal.
The construction of the transmitter part of the radio communications system for converting the intermediate frequency signal into the radio frequency signal is simplified and is made compact. The installation cost and operating cost of the radio communications system are reduced.
In a preferred embodiment of the present invention, a predetermined frequency range used by the hub station and the subscriber station is divided into a plurality of frequency channels.
Since the radio communications system employs the phase-locked loop for the low local-oscillation frequency signal only, the overall frequency accuracy of the system is improved and the utilization of frequencies is improved.